1. Field of Invention
This invention relates to optical fiber communication systems where data light is launched into the fundamental mode or into a very small set of the lowest order propagation modes of multimode graded-index fibers, resulting in very low transmission loss, low modal noise, high data security, and high data rate transmission capabilities. We have determined experimentally, as well as theoretically, that light launched into the fundamental mode or into a small set of lowest order modes of multimode graded-index fiber remains in a limited small set of lowest order modes despite severe bending perturbations of the fiber, which may occur in deployed multimode graded-index fiber transmission cable. Methods for launching the fundamental mode, or only a small set of lowest order modes in a cabled fiber, are described.
Propagation of modes within a limited small set of the lowest order propagation modes results in much less modal time dispersion, or pulse spreading, than when all modes of the multimode graded-index fiber are launched, thus permitting transmission at much higher data rates. In addition, since the lowest order modes remain in the limited small set of lowest order modes, instead of converting into radiation modes when the fiber is perturbed, data transmission attenuation is exceptionally small. Further, since the propagation modes are the lowest order modes and constrained to the center of the core, modal noise is low. Also, since data in the lowest order modes are not readily converted into radiation modes, despite severe perturbation of the fiber, high data security results.
2. Purpose
Local networks commonly use multimode graded-index fiber, which satisfies low data rate transmission requirements. Light is launched into most of the propagation modes by the light source, such as light emitting diodes. A benefit is that the large fiber core and large numerical aperture of multimode fiber allows more of the source light emission to be launched into the fiber. Another benefit is lower costs for lower bandwidth optoelectronic transceivers and peripherals. Many installed local networks currently use only multimode fibers.
In normal transmission where all modes of the multimode graded-index fiber are propagated, maximum data rate times cable length transmission capability quoted by fiber cable manufacturers is about 500 million bits per second times kilometers of fiber, or 500 Mbps-Km. The manufacturers' quoted numbers are satisfactory for many local area uses. However, much higher data rates are extremely important for future optical fiber communication networks. High data rates of many billion bits per second, or Gbps, over longer distances will be needed to transmit a manifold of new uses predicted for the future. This invention addresses that need using multimode graded-index fiber cables.
When higher data rates are required, this invention allows multimode graded index fibers in existing cables or in new cables to be used to transmit data at higher rates. Transmitting the data in a small set of the lowest order transmission modes of the multimode graded-index fiber results in high data rate capability, since modal time dispersion of pulses is reduced due the smaller number of modes, allowing shorter pulses to be transmitted. This Specification discloses how to launch the set of lowest order modes and how to use this transmission method in a variety of optical fiber communication networks.
Other significant benefits of launching only the lowest order modes, or just the fundamental order mode, into multimode graded-index fiber are that low transmission losses are incurred, modal noise is minimized, and data transmission is exceptionally secure.
3. Prior Art
Multimode graded-index fibers for communication uses were studied in the 1970's both theoretically and experimentally. Published papers on pulse broadening in multimode graded-index fiber was studied by R. Olshansky and S. M. Oaks, described in the Proc. of 4th European Conf. on Optical Communications, Geneva, p. 128, 1978, and by R. Olshansky and D. B. Keck, described in Applied Optics, vol. 15, pp. 483-491, 1975. A summary of dispersion in multimode graded-index fiber is given in a book by D. Marcuse, Principal of Optical Fiber Measurements, pp. 255-312, Academic Press, N.Y., 1981, and a brief summary by J. M. Senior, Optical Fiber Communications, Second Edition, pp. 102-135, Prentice Hall, NY, 1992.
There are over a hundred different transmission modes of both polarization in a multimode graded-index communication fiber. The precise number depends upon the radius of the fiber core, the wavelength of light used, the refractive indices of the core and cladding, and the refractive index profile of the core.
Each mode of the multimode fiber propagates through the fiber at its own characteristic group velocity. A short pulse launched into all transmission modes of the fiber at its entrance end will be spread out in time at the exit end of the fiber. The detector at the fiber exit will detect a longer pulse in time than that of the original short pulse at the entrance. The longer pulse is the cumulative sum of pulses for each mode spread in time at the exit and is defined to be due to intermodal dispersion. The spread limits the maximum bit rate transmission, or bandwidth, capability of the fiber. The limitation is also dependent on the length of fiber since the pulse spread increases with fiber length. The intermodal dispersion limitation is therefore given in terms of maximum data rate times distance. The root-mean-square pulse spread also depends upon the relative power launched into each of the modes.
A near parabolic refractive index profile for the core of the multimode graded-index fiber was found to give the largest possible data rate times distance product, by minimizing the cumulative spread of the data pulse as it propagates in all the modes. Theoretically, for a near parabolic refractive index profile, about 20 Gbps-Km should be attained, but for a number of practical reasons this capability is not attained.
Prior Patent Citations
We cite the following patents for reference. However, these do not anticipate our invention:
A. U.S. Pat. No. 5,138,675, Aug. 11, 1992, by B. A. Schofield, "Mode Scrambler as an Optical Isolator for Higher-Coherence Laser in Multi-Mode Fiber Plants". Schofield's patent Specification and claims refer to a laser to multimode fiber communication system where a mode scrambler is used at the beginning of the optical circuit in order to limit the backscattered light feedback to the laser. PA1 B. U.S. Pat. No. 4,804,248, Feb. 14, 1989 by V. A. Bhagavantula, entitled "Data Rate Limiter for Optical Transmission System". Bhagavatula's patent uses a singlemode transmission fiber. At the receiver a short multimode fiber bandwidth limiter is constructed by various alternatives in order to limit the number of modes propagated to the detector, thereby limiting the bandwidth of the link. Multimode fiber is not used as the transmission medium. PA1 C. U.S. Pat. No. 5,077,815, Dec. 31, 1991 by S. Yoshizawa et al, entitled "Apparatus for Optically Connecting a Single-Mode Optical fiber to a Multi-Mode Optical Fiber". This patent involves launching light from a laser into a pigtailed singlemode fiber which is then coupled to a multimode fiber at an angle and with an axial displacement, in order to launch high order modes into the multimode fiber. A limited number of high order modes of the multimode fiber is launched, thereby decreasing the time modal dispersion and increasing the bandwidth capability. of the multimode fiber. PA1 D. U.S. Pat. No. 5,416,862, May 16, 1995 by Z. Haas and M. A. Santura entitled "Lightwave Transmission System Using Selected Optical Modes". This patent is similar to the Yoshizawa patent. A mode coupler couples a singlemode fiber at an angle with the multimode fiber in order to launch high order modes. PA1 E. U.S. Pat. No. 5,337,380, Aug. 9, 1994, by P. Darbon and E. Grard, entitled "Method of Limiting Coupling Losses between Monomode Optical Fibers Using a Piece of Multimode Optical Fiber". This patent describes a method for connecting a multimode fiber to a second monomode fiber by "welding" the two fibers in a special way and using "adiabatic cone" fabricated to couple the light from the multimode fiber to the monomode fiber. PA1 G. U.S. Pat. No. 4,815,805, Mar. 28, 1989, by F. H. Levinson et al, entitled "Dynamic Range Reduction Using Mode Filter". The object of this patent is to provide a method and apparatus for reducing the dynamic range of an optical receiver when utilized in a distribution optical fiber system. PA1 H. U.S. Pat. No. 4,974,931, Dec. 4, 1990, by C. D. Poole, entitled "Wavelength Selective Mode Couplers". This patent involves a circularly symmetric perturbation mode coupler of a two mode fiber, where Input light in one of the modes is coupled to the second mode. Wavelength selective filtering is claimed. PA1 I. U.S. Pat. No. 5,712,937, Jan. 27, 1998, C. K. Asawa, et al, entitled "Optical Waveguide Including Singlemode Waveguide Channels Coupled to a Multimode Fiber". This patent involves a planar singlemode channel waveguide launcher for launching light from a plurality of light sources into multimode fiber, launching light into high order modes in order to perform an intrusion monitor function. The claims of the applicants' present application do not infringe the applicants' claims of the above patent. This present application is within the statutory period with respect to our other statements of the specification.